Checking integrity of cached debs

[Winston Smith <viloinuxist@yahoo.co.uk>]

Does apt-get check the integrity of cached debs before installing them?
For example, if I did 

apt-get --download-only install <package>

which downloads the package to the file

/var/cache/apt/archives/<package>.version.deb

but doesn't install it, and then sometime later, after the above file
became corrupt because of a faulty hard drive, did

apt-get install <package>

(which required no new download because there is no version change) would
the corruption be detected?

In other words, is the check done after the download, before the
installation, or both? 

Is there an easy way to check the integrity of cached debs and remove
corrupt ones? Also, is there an easy way to check the integrity of the
unchanging parts of installed packages?

Thanks (a lot).

Win